• error as undefined :

It works like when we declared a variable in the code but did not assign the value before printing the variable value.

Example:

let name;
console.log(name); //undefined

• error as not defined:

It works like when we did not declare the variable and try to call that variable.

Example :

<script>
  console.log(a);
  var a = 5;
  console.log(a);
  console.log(b);
</script>
// refference error (b) is not defined.

If the variable name which is being accessed doesn’t exist in memory space then it would be not defined, and if exists in memory space but hasn’t been assigned any value till now, then it would be undefined.